That attack appears not to have been an isolated incident, as SWIFT said Friday it has now learnt more about a second instance in which malware was used.

While SWIFT did not name the target, a report from security researchers at BAE Systems, also published Friday, pointed to a commercial bank in Vietnam as the latest victim.

The malware attacks were not directly on the transaction network or core messaging system, but instead were targeted at customer banks' secondary security controls, SWIFT said.

With its ability to transfer billions of dollars to accounts around the world in a few keystrokes, the SWIFT financial transaction network is increasingly being instrumentalized in cyber attacks on financial institutions.

While the number of cases of fraud at its customers is so far small, forensic experts believe the new discovery is part of a wider and highly adaptive campaign targeting banks, SWIFT said.

Posting on the BAE Systems threat research blog, researchers Sergei Shevchenko and Adrian Nish said what ties together the cases discovered so far is the use of an unusual file wipe function the malware uses to make deleted files unrecoverable. The function first fills the file with random characters to ensure nothing can be recovered from the sectors it occupies on disk, then changes its name to a random string before deleting it.

In both cases, SWIFT said in its latest warning, the attackers have exploited vulnerabilities in the systems banks use to initiate fund transfers, stealing banks' credentials and using them to send irrevocable fund transfer orders over the SWIFT network.

In addition, the attackers have tampered with secondary controls such as records of statements and confirmations that the banks use to recognize fraud. For example, SWIFT said, in the latest case the attackers targeted the bank's PDF reader, using malices software to modify it so as to hide traces of the fraudulent transactions in PDF reports of payment confirmations.

SWIFT asked its customers to review security controls across all their payment systems, from employee checks to cyber defenses. Banks using PDF reader applications to review confirmation messages should take particular care, it said. The asked banks to help track down the fraudsters by advising it if any incidents were discovered.